1. Field of the Invention
The present invention relates to a channel time allocation method and a wireless system using the same, and more specifically, to a channel time allocation method for efficiently supporting a variable bit rate (VBR) stream such as MPEG-2 video in a wireless personal area network and a wireless system using the same.
2. Description of the Related Art
A Wireless Personal Area Network (hereinafter, refer to as “WPAN”) is an implementation of Personal Area Network (PAN) in wireless connection. In contrast to a Local Area Network (LAN) or a Wide Area Network (WAN) that are widely known technologies, the PAN is a network owned by an individual person. That is, devices owned by the individual person are interconnected to construct the network in order to provide convenience for the individual person.
To implement the PAN with the wireless connections, Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.15 Working Group developed the WPAN for short distance wireless network standard, having four Task Groups. IEEE 802.15.1 standard is the well-known Bluetooth, IEEE 802.15.3 standard is a high rate WPAN, and IEEE 802.15.4 standard is a low rate WPAN.
The WPAN for the short distance wireless network supports various electronic devices including portable computing devices such as personal computer (PC), personal digital assistant (PDA), wireless printer, storage device, mobile phone, and set-top box. Furthermore, the high rate WPAN complies with the IEEE 802.15.3 standard or an IEEE 802.15.3a standard (hereinafter, refer to as ‘IEEE 802.15.3x’), and the IEEE 802.15.3a standard is improved from the IEEE 802.15.3 standard. The high rate WPAN enables a transmission of a real-time video, a high-quality audio, and a high-capacity file. That is, a critical field of application in the high rate WPAN is a multimedia streaming, for example, reproducing data on a TV screen from a digital camcorder or storing a video file at the PC. In order to support a multimedia Quality of Service (QoS) function, the IEEE 802.150.3x standard is configured to periodically have a Contention Free Period (CFP) and to use a Time Division Multiple Access (TDMA) scheme in the CFP.
However, a Moving Pictures Experts Group (MPEG)-2 video is featured as a variable bit rate stream having a different frame size per second. Accordingly, in supporting the VBR stream with the TDMA scheme, it may cause a waste of time in transmitting a P-frame or a B-frame to allocate a channel time fit for the size of the largest frame. On the other hand, if the channel time is not allocated according to a Peak data rate, there may not be enough time in transmitting the I-frame or the P-frame. Therefore, it is required to allocate the channel time according to the size of each frame. In addition, considering that an error rate of a wireless channel is greater than that of a wired channel and that the I-frame affects decoding of other frames, it is necessary to guarantee transmission reliability of the I-frame.
However, a current channel allocation method has a shortcoming in that it does not properly support the VBR stream such as the MPEG-2 video. That is, since the current channel allocation method allocates the channel time with a fixed sized to a superframe, a requirement for the variable time is not satisfied. Also, the current channel allocating method allocates the channel time having the superframe as a cycle, which is not in accord with a unique periodicity of the VBR stream. Hence, inefficiency may arise when the channel time is not allocated when it is necessary to transmit the frame, or when the channel time is unnecessarily allocated when it is not necessary to transmit the frame.
To solve the above problems, several methods have been proposed for the dynamic allocation of the channel time in the TDMA scheme. The first method is to increase or decrease a slot allocation after examining the current channel utilization of each user or device by a central controller. Specifically, according to the examination result, if the result is larger than the number of pre-allocated slots, the central controller increases the slot allocation, and if the result is not larger than the number of the pre-allocated slots, the central controller decreases the slot allocation. The second method is to dynamically allocate the additional channel time. Specifically, a current queue size of each user is constantly feedbacked to the central controller, and then, the central controller dynamically allocates the additional channel time with reference to the feedback. However, the first method is not appropriate to the VBR stream such as the MPEG-2 video, because the amount of the next channel time allocation is determined by the previous utilization. Also, the second method cannot guarantee to allocate the channel time within a transmission deadline of the current frame, because the additional channel time should be allocated with reference to the currently required channel time. Furthermore, these methods commonly incur an overhead because the central controller has to constantly monitor the current channel utilization or requirement of each user or device.